In order to increase the optical extraction efficiency of a nitride semiconductor light emitting device, a nitride semiconductor layer is grown on a patterned substrate.
In a related art method of fabricating a nitride semiconductor light emitting device, a sapphire substrate or a silicon carbide (SiC) substrate is selectively patterned in a predetermined shape by using a mask material. An exposed portion of the substrate is selectively dry etched. The mask material is then removed to thereby obtain a substrate with a predetermined pattern. A nitride semiconductor layer is grown on the patterned substrate.
FIG. 1 illustrates a stack structure of a related art nitride semiconductor light emitting device. As shown in FIG. 1, the related art nitride semiconductor light emitting device includes a patterned substrate 101, an n-GaN layer 103 serving as a first electrode contact, an active layer 105 emitting light, a p-GaN layer 107 serving as a second contact layer, and first and second electrodes 109 and 111 through which a bias voltage is applied.
According to the related art, a nitride semiconductor light emitting device is fabricated using a patterned sapphire substrate (PSS), and light emitted toward the sapphire substrate is again reflected to a surface of the light emitting device, whereby the optical extraction efficiency is improved. The mask material may be SiO2, Si2N4, insulating material, or metal material, which can be easily removed after an etching process.
Also, the optical extraction efficiency can be improved using a SiC substrate. Unlike the PSS, a flip chip is applied for obtaining a large-area high-power light emitting device. In order to maximize the optical extraction efficiency, a rear surface of the SiC substrate is etched. In this method, after a basic etching shape, a lateral angle and a topology are simulated, the etching is performed under the condition in which a maximum extraction efficiency can be obtained. Further, a texturing process may be performed on a surface of the etched substrate.
Also, a process of patterning the surface of the substrate and a process of patterning the rear surface of the substrate may be selectively performed according to the required performance of the applied product.
Meanwhile, according to the method in which the surface of the sapphire substrate is selectively etched and the nitride semiconductor light emitting device is grown to thereby increase the optical extraction efficiency, the substrate generally has a trench structure. The detailed trench structure of the substrate is illustrated in FIG. 2.
However, while the buffer layer of a low temperature and the GaN layer of a high temperature are grown in the surface of the trench and the non-etched surface of the sapphire substrate, the GaN layer is grown in c-axis direction within the trench and fills the trench. In the interface with the GaN layer grown in the surface, a large amount of treading dislocation (TD) is densely gathered, resulting in degradation in the reliability of the light emitting device.
This result is caused by the difference of growth rate between the surface of the trench and the surface of the sapphire substrate. Also, the surface state of the trench with predetermined depth and area is rough due to the dry etching and the surface energy of the etched sidewall is very high, the growth rate of the GaN layer is relatively high in the surface.
Also, since the growth rate in the trench is fast, when a portion meets the GaN layer grown in the sapphire substrate, a lateral growth is dominantly in progress, thus forms a void. The void is illustrated in a portion A of FIG. 3.
In addition, the GaN layer grown within the trench has relatively more crystal defect (TD, etc.) than the GaN layer grown in the sapphire substrate. Although the crystal defect density is determined by the surface roughness of the trench, the depth and the patterned shape, much crystal defect exists in the GaN layer grown within the trench. The crystal defect passes through the active region of the light emitting device and propagates up to the surface, exhibiting a high leakage current and a low reverse breakdown voltage (Vbr). Consequently, the crystal defect greatly influences the reliability of the light emitting device. The crystal defect propagated up to the surface of the light emitting device provides a pathway of a current, causing the reduction of ESD characteristic in human body mode.
According to the related art method of fabricating the nitride semiconductor light emitting device, in order to improve the optical extraction efficiency, the sapphire substrate is masked and patterned in a predetermined shape, such as a strip, a rectangle, a hexagon, and a pyramid. Then, a patterned sapphire substrate (PSS) method is widely used which performs a dry etching to a depth of 0.1˜3 μm. However, in the nitride semiconductor light emitting device using the PSS method, the surface roughness in the surface and the side of the sapphire substrate is different and the surface energy is different. Therefore, due to the difference of the growth mode of the initial nitride semiconductor, crystal defect density contained in the nitride semiconductor grown on the respective surfaces becomes different. Consequently, the crystal defect density causes the leakage current in the nitride semiconductor light emitting device and thus badly influences the reliability such as the optical power and life time in long-term operation.